"Luggage is not prohibited in the taxi."
Translation:Οι αποσκευές στο ταξί δεν απαγορεύονται.
Can you also say η αποσκευή
That's a possible expression in Greek but it means "the piece of luggage" or "the item of luggage".
It does not refer to "luggage" as a whole, which can refer to any number of items.
So in this sentence, you need the plural form αποσκευές.